Shelf stability and mantle convection on Africa’s passive margins (Part 1)

Abstract

Many clastic wedges prograding from the coast in Africa’s passive margin basins display extraordinary gravity-driven collapse structures described variously as gravity-driven linked systems, fold and thrust belts or megaslides (Butler and Turner, 2010, Scarselli et al 2016). These features form relatively slowly and are distinct from instantaneous collapse submarine landslides or Mass-Transport Complexes (MTC) that reflect sudden catastrophic shelf collapse in response to seismicity, gas hydrate destabilization or high sedimentation rates. Megaslides occur on giant scales from hundreds to thousands of square kilometres in extent, and are characterized by up-dip listric growth fault rollover systems in extensional zones, and a corresponding down-dip shortened section comprising multiple imbricate toe thrust faults and duplexes often referred to as foldand thrust belts (FTB’s). Separating the structured material from largely undeformed coherently bedded strata below, is a zone of planar, sub-horizontal detachment, or decollement. One model that seeks to explain the induced instability that initiates these features infers that the decollement surface comprises a layer that is organic-rich. Sediments prograding out over this layer eventually bury this unit to a temperature and pressure that induces early hydrocarbon generation. This increases the inter-grain pore pressure and reduces the strength of the unit, so that previously stable sediments, in a stable angle of repose begin to slide under gravity basin-ward above this decollement surface.